Terrane spotting – Testing models for the evolution of Earth’s first continents

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Archean cratons are generally dissected by steeply-dipping crustal-scale shear zones. The shear zones are commonly interpreted as terrane boundaries, separating fragments of crust with different geological histories. The accretion of terranes requires a dominance of horizontal shortening and implies some form of subduction. However, it is plausible that some of the steep shear zones may have originated from partial convective overturn of gravitationally unstable greenstone belts that sank into the deep crust. This process does not require subduction. Distinguishing between the first order geodynamic processes that formed the Archean continents is often difficult due to the long and complex history of Archean crustal rocks, which are commonly multiply deformed and overprinted by later tectonic events. In this project, we aim to test two contrasting hypotheses (terrane accretion versus later reworking of a once continuous segment of Archean crust) for the formation of the mainland Lewisian Complex, Scotland, using a combination of phase diagram modelling and in situ U-Pb, Lu-Hf, O isotope and trace element analysis of zircon. The main aim of this study is to provide a better understanding of the timing and conditions of crust formation and reworking within individual terranes, and to clarify if the crustal blocks are indeed terranes or if they represent different crustal levels of a once continuous Archean continent.